Multi-scale structural study of the Betic metamorphic complexes integrated with petrological and geochronological data

Résumé

The Betic Cordillera in southern Spain records a complex Alpine tectonic evolution derived from the Africa-Iberia convergence and the westward drift of the Alborán Domain. Several aspects concerning the tectonic processes that operated during the development of this mountain belt are still a matter of debate. In order to contribute to a better understanding of these processes, a regional-scale microstructural study of porphyroblast inclusion trails has been performed in the metamorphic complexes of the cordillera. The new data have been combined with metamorphic and geochronological data and outcrop-scale structural observations in order to integrate them in the regional tectonic framework. Inclusion trail data have been collected using 3D images obtained by scanning rock samples using the X-ray Computed microtomography (XCT) technique. This technique allows measuring the orientation of microstructural elements directly in each porphyroblast present in the sample, which supposes a major improvement with respect to traditional methods based on integration of observations made in differently oriented thin sections. A very detailed characterization of porphyroblast inclusion trails, size and shape has been performed in two samples to evaluate the suitable mechanism of porphyroblast growth, being the non-rotation model favoured by the obtained results. In the Nevado-Filábride Complex, 5 samples have been analysed through XCT to complement previous data of porphyroblast inclusion trails in the complex, which were mainly determined with thin section methods. The results confirm the presence of three sets of Foliation Intersection/Inflection Axes (FIA) oriented WNW-ESE, ENE-WSW and NNW-SSE in previously poorly studied areas of the complex. Several sets of foliations and crenulation cleavages have been studied at outcrop-scale to investigate the subdivision and internal structure of the complex on which no consensus exists. Subhorizontal high-strain zones located towards the lower part of the complex have been identified that postdate upright folding of the main foliation. These shear zones usually coincide with the boundaries of the previously defined Veleta-Mulhacén or Ragua-Calar Alto units, supporting the differentiation of these units, but a profound revision of their map-scale geometry is necessary. In the Alpujárride Complex, porphyroblast inclusion trails have been characterized through XCT and thin sections in 60 oriented samples widely distributed throughout the western and central part of the complex. Three FIA sets have been demonstrated in these rocks with similar orientations and temporal relationships (WNW-ESE, ENE-WSW and NNW-SSE) as in the Nevado-Filábride. These FIA developed mainly during the prograde evolution of the complex, but the ENE-WSW set was repeated during a late-metamorphic stage following partial exhumation of the complex. Several generations of latemetamorphic crenulation cleavages have been distinguished and interpreted to reflect alternations of contraction and extension. The regional consistency of FIA orientations in the Internal Zones and evidence for generalized subvertical and subhorizontal preferred orientations of inclusion trails shown in this thesis coincide with similar observations reported previously in other mountain belts. So far, this data has only found explanation in terms of the micro-partitioning of deformation around rigid objects, which allow them to maintain rotationally stable positions during bulk non-coaxial deformation. Crustal shortening directions indicated by the three successive FIA sets distinguished in this thesis according to this 'non-rotational' model coincide well with Paleogene-Neogene vectors of Africa-Iberia convergence and simultaneous westward displacement of the Alborán Domain. This provides an additional argument in favour of the possibility that early tectonic fabrics are preserved with their original orientations by porphyroblasts. The metamorphic conditions (7-9.5 kbar and 600-680 ºC) and age (21-18.5 Ma) of the main foliation in the Alpujárride Complex, which developed or reequilibrated during decompression, have been determined and agree well with previously published data. Microstructural evidences indicate that the WNW-ESE and NNW-SSE FIA sets developed before this foliation, although the NNW-SSE set might have been partially coeval with it. The repeated generation of ENE-WSW FIAs after NNW-SSE FIAs is confirmed by an age of 17.5 Ma obtained for monazite inclusions in garnets. This age coincides with the beginning of continental collision between the Internal and External Zones of the cordillera. In the Nevado-Filábride, a maximum age of 14.5 Ma has been obtained for a garnet porphyroblast hosting an ENE-WSW FIA and confirms a repetition of this set also in this complex. The similar orientations and temporal relationships of FIAs in both complexes suggest an equivalent kinematic evolution despite different paleogeographic positions and later exhumation of the Nevado-Filábrides. All these results demonstrate the potential of a new methodology for reconstructing the tectonic evolution of orogenic belts, whose principle ingredient is the integration of porphyroblast inclusion trails data with petrology, geochronology and outcrop-scale structural observations.